Influence Of Gap Defect States On Photoelectrochemical Properties Of Nanostructured TiO₂ And Performance Optimization

In the past 10 years,it has been found that the band gap defect states are the main factors affecting the photoelectrochemical properties of nanostructured semiconductors.However,its mechanism is still unclear.By carrying out extensive and profound investigations on both the defect states and carrier dynamics in various nanostructured TiO2 semiconductors films,this thesis is to reveal the influence mechanisms of the band gap states on the performance of nano-TiO2 photoanodes in photoelectrochemical water splitting hydrogen production,and to develop synthetic processes to high photoactive nano-TiO2 films.Field emission scanning electron microscopy(FESEM)and powder X-ray diffraction(XRD)techniques were mainly used to characterize the structure and morphology of the TiO2 films;The photoelectrochemical properties and the defect states in TiO2 were studied with various photo/electrochemical and spectroelectrochemical measurements.Particularly,electrochemical impedance spectroscopy(EIS)measurements are applied to study the defect–states-modulated carrier transport in TiO2 and transfer at TiO2/electrolyte interface.The main contents of the study mainly includes the following three parts:(1)To explore the negative influences of two common defects,that is,the surface and grain boundary defects on the photoelectrocatalytic activity of TiO2 nanoparticulate films.In order to do this,two different sintering schemes were adopted to prepare TiO2 films which bare the same nano-aggregation structure but different content of the two defects.The out come indicates that it is the grain boundary defects that exert much heavier influence on the photoelectron properties of the films,and therefore is regarded as the main demage factor for the carrier dynamics and photoelectrochemical activity of nano-TiO2 films.(2)To investigate on the basis of(1)the influence mechanisms of the defects on the photoelectrochemical properties of the films.It was disclosed that for the TiO2 films,the much greater the density of the defects,the more serious the photocurrent attenuation.A series of electron trapping experiments including electron-trapping induced dye adsorption,phenolphthalein coloration and covalent "Ti-O-P" bond hydrolysis were done.The results gave strong evidences that there exists strong electrostatic attraction between the trapped electrons and the electrolyte cations.The electrostatic interactions will increase the energy barrier for electron detrapping and consequently increase electron transport resistance and recombination,which is believed to be the main reason to the general photoactivity decay of nano-TiO2 films.(3)To develop based on(1)and(2),High temperature treatment strategy to improve photoelectrochemical activity during preparation of single crystalline TiO2 nanorod array films through sol-gel/hydrothermal process.High temperature sintering can significantly reduce the defects,thus optimize the carrier transport dynamics.This thesis can be a good reference to theoretical investigations on the influence mechanisms of band-gap trap states of nanostructured semiconductors on photoelectrochemical properties,and to synthetic researches to high-performance semiconductor photocatalysts and photoelectrodes.